The present invention generally relates to optical encoders and more particularly to optical encoders for measuring and analyzing rotary and linear motions. The present invention includes a linear formation of three photodiodes for detecting light passing through a raster, wherein the photodiodes are also specifically spaced from the raster.
Optical encoders function to register rotary angular motion or linear motion as well as direction of a moving body. Encoders generally comprise a light emitting system; a slit plate, raster or scattering arrangement, plate, sheet, or ruler (herein referred to as a raster); and a detecting arrangement positioned to receive light reflected off of or passing through the raster arrangement. The raster is attached to a rotating body, such as a shaft, for which the speed and direction are sought. The emitting system generally comprises a light emitting diode or laser diode with a lens arrangement optically coupled thereto. A light bundle may also be used to transport the light from the source to the raster arrangement. The detecting arrangement detects the light as modulated by the raster and generates information in response to the detection. The information is indicative of the speed and direction of the shaft to which the raster is connected.
In current encoders, the detecting arrangement is made or otherwise arranged to be evenly illuminated. Four photodiodes are typically employed and evaluation logic is electronically coupled to the detecting arrangement. The photodiodes are arranged linearly and parallel or tangential to the raster arrangement. The space between the photodiodes are one quarter of the raster arrangement period. The signals generated by adjacent photodiodes are phase shifted by ninety degrees respectively. The evaluation logic then determines the linear or angular speed of the raster arrangement based on the output of the photodiodes.
U.S. Pat. No. 4,654,525 sets out an optical encoder arrangement for detecting rotational movement of a circular raster. The raster Is concentrically arranged about a central axis and includes periodical openings moving between a light source and a linear array of four photodiodes. The photodiodes thus detect the rotational motion of the raster by analysis of light passing through the raster openings and made incident upon the photodiodes. The opening within the raster is formed in such a way that the width of opaque or transparent areas corresponds to the combined lengths of two adjacent photodiodes and spacing therebetween. By this arrangement, four adjacent signals are produced by the photodiodes, except that the signals are each out of phase by ninety degrees thereby producing an quadrature signal. The quadrature signals are fed passed adders and comparators so as to arrive at two quadrature signals out of phase by ninety degrees. The two signals are indicative of the rate and phase proportion of the rotary direction and speed of the raster.
U.S. Pat. No. 4,691,101 sets out a similar optical encoder. Herein several linear arrays of four photodiodes are successively arranged and the output of those diodes, in phase, are further electrically coupled. Alternative arrangement schemes are set out for arrangement of the photodiodes.
Heretofore, it is a given in the art that four semiconductor photodiodes are to be employed in an optical encoder. The area of each photodiode (along with some spacing) is set to half the area of the opening within the raster and further separately electrically biased. As such, this arrangement entails high manufacturing and engineering costs as well as high consumption of semiconductor chip area.
Accordingly, it is an object of the present invention to set out an optical encoder for detection and analysis of linear or angular motions which can be produced at reduced engineering and manufacturing costs, be designed for high replication, require reduced area of the semiconductor chip area, while still remaining relatively immune to light intensity fluctuations.
These and other objects of the present invention are accomplished by an optical encoder comprising light emitting means for generating light; a raster for modulating said generated light, said raster comprising a plurality of periodically positioned first and second formations, said first formations being opaque and said second formations being transparent, and said raster being attached to a moving element; light detection means for detecting light modulated by said raster, said light detection means comprising at least one set of three light detecting elements having light receiving areas being positioned in parallel to said raster, said elements generating an output responsive to detected light; and evaluating means electrically coupled to said detecting elements, said evaluating means comprising evaluating elements for receiving said detecting means output and generating an evaluating means output indicative of speed and direction of moving element.
The optical encoder according to the present invention includes three photodiodes. Herein, a reference signal is produced by a suitable evaluation circuit and used to compensate for light intensity fluctuations. The circuit includes an adder which receives the output of the first and third photodiodes. A signal level reduction component is connected to the output of the adder so as to produce a reference signal at its output. The reference signal is then fed into at least two comparators which compare the reference signal with the output of the first and second photodiodes. The outputs of the comparators are quadrature signals which are indicative of the angular speed and direction of the raster. In addition, the signals are out of phase by ninety degrees. The signal level reduction means may be a divider, lowering the adder output by a factor of two. Fluctuations of light intensity do not adversely effect the measuring capability of the present arrangement.
The present arrangement also employs sets of three photodetectors in place of the four used in the prior art. The photodetectors, which may comprise photodiodes, are spaced apart, such that the length of two light receiving areas of two adjacent photodiodes is about the same as a length of an opaque or transparent formation in the raster. The combined length of the light receiving areas may correspond to half or three quarters of the period of the raster. Given the linear arrangement of the photodetectors, the compact space advantage of the prior art is preserved. Likewise, the at least one set of three photodetectors may be arranged on a single substrate with the circuitry for the evaluation step formed thereon or therein. One advantage over the prior art lay in the reduction of at least one photodiode (from four to three), thereby resulting in decreased manufacturing and engineering costs. Additional advantages will become clear from the continued description of the present invention below.